It is well known in the art that the flux and energy of ions is important in determining how a plasma treatment will affect a workpiece. There exist several different methods of adjusting the ion energy during plasma treatment, none of which are generally applicable.
Coburn and Kay, J. Appl. Phys. 43(12):4965-4971 (1972) (cited as Coburn) describes a geometrically based Direct Current (DC) offset effect that depends upon a Radio Frequency (RF) voltage being applied across electrodes immersed in a plasma which have different surface areas. The method of Coburn is commonly used in the semiconductor industry for the manufacture of integrated circuits, and is implemented by placing a silicon wafer upon an electrode immersed in a plasma. Typically, the wall of the vacuum chamber functions as a second electrode and has a significantly larger surface area than the electrode with the wafer. This causes an increase in the average energy of ions striking the silicon wafer. This technique is not applicable when the workpiece has a very large surface area, because the vacuum chamber would be prohibitively large. An example would be a photovoltaic module manufactured using thin films. These modules typically have a surface area greater than a square meter.
There have also been methods developed where multiple RF voltages are applied to one or more electrodes. Boyle, Ellingboe, and Turner, J. Phy. D: Appl. Phys 37:697-701 (2004) (cited as Boyle) describes how a low frequency is used to modulate the ion energy, and a large frequency is used to control plasma production. The abstract of Boyle states that: “In such discharges, a low frequency component couples predominantly to the ions, while a high frequency component couples predominantly to electrons. Thus, the low frequency component controls the ion energy, while the high frequency component controls the plasma density. Clearly this desired behavior is not achieved for arbitrary configurations of the discharge, and in general one expects some unwanted coupling of ion flux and energy.”
The method described in Boyle is commonly used in the semiconductor industry, and has been only partially successful in allowing the independent control of ion flux and energy. The method described in Boyle is also difficult to apply to large workpieces due to standing wave effects caused by the high frequency RF voltage.